CD4+ T cell phenotypes can be defined according to the pattern of cytokines secreted (Abbas, A. K., et al. (1996) Nature 383, 787-793). Type 1 immunity relies on the generation of Th1 cells, whose hallmark cytokine is interferon-gamma (IFN-γ) (Szabo, S. J., et al. (2003) Annu. Rev Immunol 21: 713-758). Th2 cells produce a different spectrum of cytokines including interleukin-4 (IL-4) and interleukin-5 (IL-5) and are important in the generation of Type 2 immunity (Abbas, A. K., et al. (1996) Nature 383, 787-793). T-bet is a T-box transcription factor essential to Th1 cell generation and effector function (Szabo, S. J. et al. (2000) Cell 100, 655-669). Recently, its expression has also been described in NK cells, dendritic cells and CD8+ cells (Szabo, S. J. et al. (2002) Science 295, 338; Lugo-Villarino, G., et al. (2003) Proc. Natl. Acad. Sci. U.S.A 100, 7749-7754; Townsend, M. J. et al. (2004) Immunity. 20, 477-494). T-bet directly transactivates the IFN-γ gene in CD4+ T cells and increases the expression of IL-12 receptor β chain on activated cells. Indeed, a positive feedback loop is observed, since STAT1 downstream of the IFN-γ receptor activates T-bet expression, which further serves to increase IFN-γ secretion (Robinson, D. S. & O'Garra, A. (2002) Immunity. 16, 755-758). The strong transactivation of IFN-γ by T-bet makes it difficult to dissect which genes are targets of T-bet and which lie downstream of IFN-γ, as this cytokine is known to induce the expression of many hundreds of genes. When overexpressed in fully polarized Th2 cells, T-bet can reverse their lineage commitment and induce Th1 specific genes, particularly IFN-γ and its known targets (Szabo, S. J. et al. (2000) Cell 100, 655-669; Szabo, S. J. et al. (2002) Science 295, 338; Lametschwandtner, G. et al. (2004) J Allergy Clin. Immunol 113, 987-994). Animals deficient in T-bet demonstrate a marked reduction in severity to a number of inflammatory diseases, including SLE, colitis, diabetes, hepatitis and arthritis, with a number of different abnormalities in effector function described in CD4+ and CD8+ cells (Peng, S. L., et al. (2002) Proc. Natl. Acad. Sci. U.S.A. 99, 5545-5550; Neurath, M. F. et al. (2002) J. Exp. Med. 195, 1129; Juedes, A. E et al. (2004) J Exp. Med 199, 1153-1162; Hultgren, O. H., et al. (2004) Microbes. Infect. 6, 529-535; Siebler, J. et al. (2003) Hepatology 38, 1573-1580). However, it has been difficult to dissect the precise mechanisms of this protection, as many cell types in the immune system express T-bet Furthermore, given its function as a master regulator of T cell lineage commitment, it is likely to direct the transcription of many genes involved in both cytokine production and other effector pathways.
Effector Th1 and Th2 cells differ profoundly in their migratory properties (Austrup, F. et al. (1997) Nature 385, 81-83; Xie, H., et al. (1999) J Exp. Med. 189, 1765-1776; Iezzi, G., (2001) J. Exp. Med. 193, 987-993; Rot, A. & von Andrian, U. H. (2004) Annu. Rev Immunol 22:891-928). Th1 cells migrate to sites of inflammatory immune responses, whereas Th2 cells migrate predominantly to mucosal sites in the settings of allergy or helminth infection. E- and P-selectin ligands are expressed mainly on Th1 cells, being absent on naive T cells and greatly reduced on Th2 cells (Lim, Y. C. et al. (1999) J. Immunol. 162, 3193-3201). The initial step in Th1 cell recruitment is binding to P- and E-selectin expressed on activated vascular endothelium, interactions mediated mostly by the leukocyte ligand P-selectin glycoprotein ligand-1 (PSGL-1) (Hirata, T. et al. (2000) J. Exp. Med. 192, 1669-1676; Yang, J. et al. (1999) J. Exp. Med. 190, 1769-1782). Interestingly, PSGL-1 undergoes further enzymatic post-translational modification including core-2-glycosylation, facosylation and tyrosine sulfation, all of which are required to produce a functional selectin ligand (McEver, R. P. & Cummings, R. D. (1997) J. Clin. Invest 100, S97-103). Upregulation of these selectin ligands has thus far been ascribed to the actions of IL-12 on activated lymphocytes, largely in a STAT4 dependent manner (Lim, Y. C. et al. (1999) J. Immunol 162, 3193-3201; Lim, Y. C. et al. (2001) J Immunol 167, 4476-4484). Chemokines also play critical roles in T cell recruitment by mediating both the transition from selectin-dependent rolling to integrin-mediated firm adhesion (Campbell, J. J. et al. (1998) Science 279, 381-384), as well as the subsequent locomotion and transendothelial migration of T cells (Cinamon, G., et al. (2001) Nat. Immunol. 2, 515-522). Differential expression of chemokine receptors plays a key part in the process of T cell migration to inflammatory sites. The chemokine receptors CXCR3 and CCR5 are thought to be responsible for the specific recruitment of Th1 cells to inflammatory sites. Other chemokine receptors (e.g., CCR4, CCR10 and CCR9) have also been described to mediate tissue specific homing, although not necessarily in a Th1 specific manner (Syrbe, U., et al. (1999) Springer Semin. Immunopathol. 21, 263-285). In contrast, other lymphocyte adhesion molecules implicated in the adhesion cascade, in particular the β1 and β2 integrins, LFA-1 and VLA4, have not been implicated in the specific recruitment of Th1 cells, rather of activated lymphocytes in general. The regulation of cellular trafficking is therefore crucial to an effective immune response.
The resistance of T-bet-deficient (T-bet−/−) mice to inflammatory diseases is characterized by a striking lack of T cell infiltration at pathologic sites (Neurath, M. F. et al. (2002) J. Exp. Med. 195, 1129; Juedes, A. E et al. (2004) J Exp. Med 199, 1153-1162). However, the disease models studied in this context have relied critically upon intact effector function for expression of disease and as such it has been impossible to study T cell tricking in isolation. Identification of a mechanism by which T-bet directly modulates T cell recruitment to sites of inflammation would allow for modulation of the T cell recruitment and inflammation and would be of great benefit.